Tilting transmission gearbox comprising a pivoting connection with plain bearings

ABSTRACT

A transmission gearbox ( 1 ) has a casing ( 2 ) that tilts around a tilting axis (B-B) via two bearings ( 19 ). Each of the bearings includes: a fixed part ( 20 ) having a sleeve ( 20   a ); and a trunnion ( 21   a ) which belongs to a pivoting part ( 21 ) and which pivots around the sleeve, the pivoting part being solidly connected to the casing ( 2 ). A shouldered wear ring ( 22 ) is disposed between the sleeve ( 20   a ) and the trunnion ( 21   a ) of each bearing ( 19 ). The invention is suitable for use in the pivot mounting of a power transmission gearbox ( 1 ) that is used to rotate the tilting rotor ( 14 ) of a convertible aircraft.

The invention relates to a tilting transmission gearbox and morespecifically to a notched tilting power transmission box swiveling abouta tilting axis to drive in rotation at least one device driven about therotation axis which must be capable of swiveling around the tiltingaxis, almost perpendicular to the axis of rotation of the driven device,with said transmission box comprising a casing mounted to tilt withrespect to and preferably between two fixed and rigid side parts of asupport, by two coaxial bearings around said tilting axis and spaced atsome distance from one another along this tilting axis.

The tilting power transmission gearbox according to the invention may beused, in a non-limitative manner, as a power transmission box onaircraft having an adjustable propulsion system, such as dirigibleballoons or aircraft of the <<convertible>> type or yet again, as apower transmission box on wind generators.

In general, the tilting power transmission gearbox according to theinvention may be used in any adjustable propulsion on power generationsystems by rotation, which, to operate, require a small axial deflection(in the direction of the tilting axis).

In particular, as an application for which the tilting powertransmission gearbox according to the invention is of particularinterest to the applicants is a transmission gearbox that can be usedfor driving in rotation, from at least one source of power, such as amotor-propulsion unit, for instance of the turbomotor type, at least onetilting rotor of a “convertible” type aircraft which can operate inaircraft mode or in helicopter mode and whose drive shaft swivels abouta tilting axis so as to be able to change from one position, in whichthe rotor operates like an aircraft propellor, to fly in aircraft mode,to another position in which the rotor operates like a helicopter rotorfor the aircraft to fly in the helicopter mode.

A tilting power gearbox for an aircraft of the convertible type withtilting rotors is described more particularly in FR 2 791 319 and FR 2791 634, which should be referred to for more details on the subject.

However, it should be noted that these two patents describe aconvertible aircraft with tilting rotors of a type in which the driveshaft of each rotor is driven in rotation about its own axis by a fronttilting reducing assembly of one respectively of the two powertransmission gearboxes each including a fixed rear reducing assemblyconnected to the corresponding front reducing assembly and,respectively, to one or two motors (turbomotors), each supportedrespectively one of the two fixed wings of the aircraft and aninterconnection shaft connecting the two transmissions to drive the tworotors in rotation by either of the two motors, in the event of theother motor failing.

The shaft of each rotor and the corresponding power transmission as wellas the corresponding motor, are housed in respectively one of twoarticulated casings, each provided with a front part tilting about thetilting axis, and a rear fixed part, attached respectively to one of thetwo fixed wings of the aircraft and in which are housed thecorresponding motor and at least, partly, the rear reducing assembly ofthe corresponding transmission whose front reducing assembly,constructed like the main helicopter transmission gearbox, and the shaftof the rotor corresponding to it housed in the front tilting part of thecasing are assembled to tilt with this front casing part with respect tothe fixed rear part of the casing and the corresponding facing wing.

In this application and with this architecture for each powertransmission gearbox, the tilting link of front tilting reducer assemblyoperates only through an angular tilting sector limited to approximately110° with a low speed of rotation included between approximately 1 andapproximately 2 rpm and under a static load.

Accordingly, the two bearings through which the front tilting reducerassembly casing, or the tilting power transmission gearbox is mounted totilt on the support, i.e. are subject to a <<false Brinnel effect>>which is particularly sensitive when the bearings are of the rollingtype, such as needle or roller bearings.

The basic problem of the invention is to reduce or to eliminate entirelythis <<false Brinnel effect>> of the bearings and more generally, topropose a tilting setup for the power transmission gearbox on itssupport, better suiting the various demands of the embodiment of thetechnique and more specifically offering better dynamic behaviour of thepropulsion system in which this type of integrated power transmissiongearbox is included, in particular by reducing the pressures in theaforementioned bearings, and the friction during the deflection of thispropulsion system about the tilting axis.

To achieve this, the tilting power transmission gearbox of the typepresented above is characterised in that the two bearings are plainbearings each of which includes:

-   -   a fixed part integral with respectively one of the two side        parts of said support, and including a sleeve, more or less        cylindrical, approximately coaxial with said fixed part of the        other bearing, about the tilting axis,    -   a swiveling part integral with said tilting casing and including        a more or less cylindrical annular trunnion, mounted to swivel        about said sleeve of the fixed part of said bearing and more or        less coaxial with the corresponding trunnion of the swiveling        part of the other bearing, and    -   a wear ring, including a cylindrical tubular part, engaged        axially between said sleeve of said fixed part and said trunnion        of said mobile part and a radial collar, protruding radially        toward the outside of said tubular part and with respect to its        axis, integral with an axial end of said tubular part which is        turned towards said corresponding lateral part of the support.

In an advantageous embodiment, the casing is mounted to tilt between thetwo fixed and rigid lateral parts of the support and the sleeves of thefixed parts of the two plain bearings, extending towards one another,between said parts of said support.

In order not to generate any resisting couple when the transmissiongearbox tilts, it is an advantage for the wear ring to be constructed onthe basis of a material having a low friction coefficient, for instancePTFE,

In addition, for better tilting set up of the casing thanks to thebearings, the wear ring of at least one bearing, and preferably of both,is secured to the swivelling part of said bearing.

The attachment of the wear ring to the swiveling part of thecorresponding bearing is ensured, in a manner that benefits fromsimplicity, by at least three screws inserted in a direction more orless parallel to the tilting access, passing through said radial collarand attached to the swiveling part of said corresponding bearing,

In addition, to decrease the Hertz pressures on the bearings, thetubular parts of the wear ring on at least one bearing, and preferablyon each of them, will have a radial internal bearing surface the shapeof which is slightly domed and convex toward the tilting axis while thelocation and amplitude of the domed convex shape will be such that theinternal radial bearing surface is more or less cylindrical having acircular section and a load, more particularly static, taken up by saidtilting casing.

In addition, to also decrease the pressure peaks at the axial ends ofthe inside radial bearing surface of the wear ring of at least onebearing, and preferably on each of them, this internal radial bearingsurface will have, at least at one of its two axial ends, and preferablyat each of the two axial ends, an arrangement forming a chamfer or aninclined face.

Finally, for application to a convertible aircraft with tilting rotors,the bearings will be solicited by an angular sector with a limitedtilting movement of approximately 110° with a low speed of rotationbetween approximately 1 and approximately 2 rpm, with said tiltingcasing bearing with a static load about the tilting axis.

The invention also refers to an aircraft of the convertible type capableof operating in aircraft mode or helicopter mode, including at least onetilting power transmission gearbox designed to drive in rotation atleast one rotor about a rotation axis from at least one power source,such as a motor-propulsion unit, with said rotor mounted to tilt withsaid transmission gearbox about a tilting axis more or lessperpendicular to said axis of rotation to change between aircraft andhelicopter mode, with the aircraft characterised in that said tiltingpower transmission gearbox is a transmission gearbox according to theinvention as defined above.

Other characteristics and advantages of the invention will be revealedin the description given below on a non-imitative basis, of an exampleof an embodiment described with reference to the attached illustrationsin which:

FIG. 1 is a partial schematic sectional view of a tilting powertransmission gearbox according to the invention,

FIG. 2 is a partial schematic transversal sectional view on a largerscale of a bearing for the transmission gearbox of FIG. 1,

FIGS. 3, 4 and 5 are axial sectional half-views of methods of obtainingthe wear ring for the bearing of FIG. 2, with an associated curverepresenting the Hertz pressure and the pressures at the ends of thering, and where

FIGS. 6, 7 and 8 are schematic representations of other axialhalf-sections of the wear ring with alternative methods for obtainingthe internal radial bearing surface domed aspect.

Tilting power transmission gearbox 1 shown in FIG. 1 includes a tiltingcasing 2 housing a reducing assembly which, in the aforementionedapplication, driving in rotation a tilting rotor of a convertibleaircraft as described in the two documents of the aforementioned patentdocuments, establishes the front tilting reducer assembly, arranged likea main helicopter transmission gearbox, of a transmission connecting amotor to this rotor and an interconnection shaft to another similartransmission, said transmission including a non-tilting motor reducingassembly, supported by a structure secured to the aircraft structure,for instance the structure of a rear fixed part of a motor casing, thefront tilting part of which encloses power transmission gearbox 1.

In the example shown here, the reducer assembly housed in tilting casing2 has two reducing stages including one input stage 3 which is areducing stage with a pair of spiro-conical driving gears including aprimary conical driving gear 4 driven in rotation about the tilting axisB-B of transmission gearbox 1, by a coaxial shaft 5, itself driven inrotation from the non-tilting rear reducer assembly (not shown) with theteeth of conical driving gear 4 meshing with those of a secondaryconical driving gear 6, integral in rotation, about an axis A-A, more orless perpendicular to the tilting axis B-B of a coaxial shaft 7interconnecting with the first reducing stage or input stage 3 and thesecond reducing stage or output reducing stage 8.

This reducing stage 8 is an epicyclic stage including a planet pinion 9integral with shaft 7 and conical driving gear 6 in its rotation aboutthe A-A axis and the toothing of which meshes with the toothing ofplanet driving gears 10 mounted to rotate about axes parallel to the A-Aaxis on a planet-holder 11, with the toothing of planet driving gears 10also meshing with the inner toothing of a peripheral ring gear 12attached to the inside of tilting casing 2. Planet holder 11 is integralin coaxial rotation about the A-A axis with a rotor mast 13, guided inrotation within casing 2 by bearings, not shown, and the other end ofwhich protrudes out of casing 2 and is integral with hub 15 supportingthe blades 16 of rotor 14, driven in rotation about the A-A axis, whichis the axis of rotation of the rotor.

This rotor 14 and the power transmission gearbox 1 comprising reducerstages 3 and 8 are mounted to tilt around the tilting axis B-B withcasing 2, mounted to tilt about this axis B-B between two fixed andrigid side parts 17 of a support, generally designated under reference18, itself fixed and rigid, for instance, in the case of the applicationin question, a support rigidly linked with the structure of the rearfixed part of the engine casing, itself integral with the structure ofan aircraft wing (represented schematically as P in FIG. 1).

The swivelling assembly of casing 2 about the B-B taxes and between thetwo lateral parts 17 of support 18 is provided by two coaxial plainbearings 19 turning about the B-B tilting axis and spaced away from oneanother along this axis B-B so that each bearing 19 is adjacent torespectively one of the lateral parts 17 of the fixed support.

As is also shown in detail and at a larger scale in the axial halfsection of FIG. 2, each bearing 19 includes a fixed part 20 integralwith the lateral part of adjacent support 17 and includes a sleeve 20 a,more or less cylindrical with a circular section and approximatelycoaxial with sleeve 20 a of fixed part 20 of the other bearing 19, aboutthe tilting axis B-B, each sleeve 20 a extending towards the othersleeve 20 a from the corresponding fixed lateral part 17. Each fixedpart 20 also has an annular shoulder 20 b, radial (with respect to theB-B axis) and protruding toward the outside of sleeve 20 a, and that maybe attached to the fixed lateral part of corresponding support 17 asschematically shown by the dotted lines in FIG. 2, or incorporated intothis part 17.

Each bearing 19 also includes a swivelling part 21, integral withtilting casing 2, in the part of the latter extending under the ringgear 12 with this swivelling part 21 including an annular trunnion 21 awhich is more or less cylindrical and circular in section, at least onits internal radial bearing face, and which is more or less coaxial withtrunnion 21 a of the swivelling part 21 of the other bearing 19.

In each bearing 19, trunnion 21 a of swivelling part 21 is mounted toswivel about the sleeve 20 a of fixed part 20 and each bearing 19 alsoincludes a wear ring 22 obtained on the basis of material with a lowfriction coefficient, for instance PFTE, interposed between the fixedpart 20 and swivelling part 21 of the corresponding bearing 19 so as notto generate any resistant couple during the tilting of casing 2.

Each wear ring 22 has a tubular part 22 a, more or less cylindrical witha circular section, engaged axially between sleeve 20 a and trunnion 21a belonging respectively to fixed part 20 and swivelling part 21 of saidbearing 19. Each wear ring 22 also has a radial collar 22 b which is anannular collar protruding radially toward the outside of tubular part 22a corresponding to it, and with respect to the latter, integral with theaxial ends of tubular part 22 a that is turned towards the correspondinglateral part of support 17.

To facilitate assembly and improve the operation of bearing 19, eachwear ring 22 is attached to swivelling part 21 of corresponding bearing19 and, as shown in FIG. 2, this attachment is obtained, for instance,by three screws 32 that are screwed in more or less parallel to the B-Btilting axis through part of collar 22 b of said ring 22 which isradially positioned toward the outside so that the end of the stem ofscrews 32 is screwed into the radial shoulder 21 b connecting trunnion21 a of swivelling part 21 of bearing 19 to the remainder of tiltingcasing 2 with the heads of screws 32 embedded in the radial face in theouter axial position (towards the outside of bearing 19) of collar 22 b.

In the application mentioned above of a tilting transmission gearbox 1driving the tilting rotor of a convertible aircraft, the swivelling linkprovided by bearing 19 between tilting casing 2 and fixed support 18working only in a small angular sector limited to approximately 110° onwhich bearings 19 are solicited with a speed of rotation about the B-Btilting axis that is relatively small, between approximately 1 andapproximately 2 rpm whereas tilting casing 2 and therefore bearings 19are statically loaded. Since bearings 19 are affected by a “falseBrinnel effect”, their plain structure, according to the invention, isfar more advantageous than rolling housing bearings and, in addition,the use of plain bearings 19 with a wear ring 22 makes it possible tooptimise the shape of the bearing surface in the inside radial positionon the tubular part 22 a of the wear ring 22, the internal radialbearing surface on which the swivelling link is made, thus reducing theHertz pressures and the pressure peak at least at one end of the axialends of this internal radial bearing surface.

FIGS. 3 to 5 are a transversal sectional schematic representation ofthree internal bearing surface ring shapes superimposed on curvesrepresenting, depending on the axial position on ring 22, the amplitudeof the Hertz pressures and the pressure peaks at the axial ends. FIG. 3corresponds to ring 22 whose internal radial bearing surfaces and outerbearing surfaces of tubular part 22 a are perfectly cylindrical with acircular section and corresponding Hertz pressure curve 34 showsconsiderable pressures with two pressure peaks at 35 and 36 at the axialends of tubular section 22 a.

FIG. 4 shows an initial variant of the wear ring 22′ in which bevels 37and 38 shaped as a chamfer or a truncated inclined section have beenmachined at each axial end of the internal radial bearing surface oftubular part 22′a. The corresponding Hertz pressure curve 39 representslower pressure that is less variable according to the axial positionthan pressure curve 34 of FIG. 3, with, in addition, pressure peaks 40and 41 that are highly attenuated at the axial ends.

The second variant of the wear ring 22″ of FIG. 5 includes similarbevels 37 and 38 as ring 22′ of FIG. 4 with, in addition, a domedcentral part 42, convex towards the inside of tubular part 22″a of thisring 22″ with “doming” amplitude b of approximately several micrometers.It can be seen that pressure curve 43 is of almost constant amplitudeover the entire axial dimension of ring 22″ including at the axial ends,said Hertz pressure amplitude also being less than those of curve 39 inFIG. 4, which themselves are substantially less than the pressures givenby curve 34 of FIG. 3.

Wear ring 22 of FIG. 2 is therefore, preferably, a ring like 22″ of FIG.5 in which, in addition, the location and amplitude b depend on theloading applied to corresponding bearing 19 and the deformations of thesurrounding parts with the apex of the domed part possibly located moreare less at the middle of the axial length “I” of tubular part 22″a ofring 22″, as shown schematically in FIG. 7, or yet again, offset towardthe axial end on the side of radial collar 22 b, as shown schematicallyin FIG. 6 or to the contrary, offset toward the other axial end of thistubular part 22″a, as shown in FIG. 8. The choice between thesedifferent shapes and positions of the convex domed part of the internalradial bearing surface of a ring like 22″ is made so that under load, inparticular static as supported by tilting casing 2 and thereforecorresponding bearing 19, this internal radial part takes on a more areless cylindrical section circular shape which will procure a Hertzpressure curve as shown in curve 43, relatively flat with a smallamplitude, as shown in FIG. 5, without a pressure peak at the axialends.

The use of plain bearing 19 equipped with wear rings like 22, 22′ andabove all 22″, will obtain a considerable decrease in friction duringswivelling about the B-B tilting axis, while substantially limiting theaxial deflections of the propulsion system including a rotor 14 and anassociated tilting power transmission gearbox 1, and a considerabledecrease in the pressure in the associated bearings 19.

This produces better dynamic behaviour of the propulsion system and atbearings 19, eliminates the “false Brinnel effect”.

1. A tilting power transmission gearbox (1), designed to drive inrotation at least one driven part (14) about an axis of rotation (A-A)being able to swivel about a tilting axis (B-B), more or lessperpendicular to said axis of rotation (A-A) of said driven device (14),such as the tilting rotor of a convertible aircraft with saidtransmission gearbox (1) having a housing (2) mounted to tilt withrespect to two fixed and rigid lateral parts (17) of a support (18) viatwo coaxial bearings (19) about said tilting axis (B-B) and at intervalsfrom one another along said tilting axis (B-B), characterised in thatthe two bearings (19) are plain bearings each of which includes: a fixedpart (20), integral with respectively one of the two lateral parts (17)of said support (18), and including a sleeve (20 a) that is more or lesscylindrical and that is approximately coaxial with sleeve (20 a) of saidfixed part (20) of the other bearing (19), around tilting axis(B-B). aswivelling part (21) integral with said tilting casing (2) and includinga more or less cylindrical annular trunnion (21 a) mounted to swivelabout said sleeve (20 a) of fixed part (20) of said bearing (19) andmore or less coaxial with the corresponding trunnion (21 a) of theswivelling part (21) of the other bearing (19), and a wear ring (22)comprising a cylindrical tubular part (22 a), engaged axially betweensaid sleeve (20 a) of said fixed part (20) and said trunnion (21 a) ofsaid mobile part (21), and a radial collar (22 b), protruding radiallytoward the outside of said tubular part (22 a) and with respect to itsaxis, and integral with an axial end of said tubular part (22 a) whichis turned towards said corresponding lateral part (17) of support (18).2. A tilting power transmission gearbox according to claim 1,characterised in that said casing (2) is mounted to tilt between said 2lateral parts (17) of said support (18), and sleeves (20 a) of the fixedparts (20) of said plain bearings (19) extending towards one anotherbetween said lateral parts (17) of said support (18).
 3. A transmissionbox according to claim 1, characterized in that said bearing ring (22)is constructed on the basis of a material having a low frictioncoefficient, for instance, PTFE.
 4. A transmission gearbox accordingclaim 1, characterized in that the wear ring (22) of at least onebearing (19) is attached to said swivelling part (21) of said bearing(19).
 5. A transmission gearbox according to claim 4, characterised inthat the attachment of said wear ring (22) to said swivelling part (21)of corresponding bearing (19) is provided by at least three screws (32)running in a direction more almost parallel to tilting axis (B-B),through said radial collar (22 b), and screwed into said swivelling part(21) of said corresponding bearing (19).
 6. A transmission gearboxaccording to claim 1, characterised in that said tubular part (22″a) ofsaid wear ring (22″) of at least one bearing (19) has an internal radialbearing surface whose shape is slightly domed and convex towards tiltingaxis (B-B), with the location and amplitude (b) of the convex domedshape being such that the internal radial bearing surface takes on aform more or less cylindrical and having a circular section under theload, essentially static, supported by said tilting casing (2).
 7. Atransmission gearbox according to claim 6, characterised in that saidthe internal radial bearing surface of said wear ring (22″) of at leastone bearing (19) in presence, has at least at one of its two axial ends,a bevel (37, 38) shaped as a chamfer or an inclined surface.
 8. Atransmission gearbox according to claim 1, characterized in that saidbearings (19) are solicited in an angular sector having a limitedtilting angle of approximately 110° with a low speed of rotation ofbetween approximately 1 and approximately 2 rpm, of said tilting casing(2) loaded statically about tilting axis (B-B).
 9. An aircraft of theconvertible type capable of operating in aircraft mode or helicoptermode including at least one tilting power transmission gearbox (1)designed to drive in rotation at least one rotor (14) about an axis ofrotation (A-A), from at least one source of power, such as a motorpropulsion unit, said rotor (14) being mounted to tilt with saidtransmission gearbox (1) about a tilting axis (B-B) more or lessperpendicular to said axis of rotation (A-A) to change between aircraftmode and helicopter mode, characterised in that said tilting powertransmission gearbox (1) is a transmission gearbox according to claim 1.10. A transmission box according to claim 2, characterized in that saidbearing ring (22) is constructed on the basis of a material having a lowfriction coefficient, for instance, PTFE.